1. Field of the Invention
The present invention relates to a bitumen composition having improved resistance to deformation under load and decreased tackiness. The bitumen composition is particularly useful as a backing for carpet tile.
2. Description of the Prior Art
Carpet tiles have been conventionally manufactured by coating the back surface of a carpet material having a fibrous wear surface with one or more coatings of a heavy duty carpet backing composition. Such carpet backing compositions, all of which in general have been known for years, include amorphous polyolefins, bitumens, polyvinyl chloride and polyurethane blends. Because of its relatively low cost and ready availability bitumen is an attractive potential primary component for carpet backing compositions. Bitumen, however, has suffered from several deficiencies that have made it unsuitable for use in high quality carpet tile products. Firstly, bitumen backed carpet tiles have suffered from inadequate resiliency. Under high load conditions, e.g., under the legs of chairs or tables or when exposed to foot traffic that includes high heel shoes, permanent indentations or other damage may occur that will have a negative effect on the "lay" or dimensional stability of the sectional carpet tile. The reason for this is plastic deformation of the backing composition under the influence of the load. This phenomenon is known as "cold flow". The use of polyolefin additives to stiffen the backing composition and decrease cold flow has not met with success in the prior art, due in part to incompatability between the polyolefin and bitumen leading to phase separation.
Another disadvantage of bitumen backed carpet tiles is the undesired tackiness of the exposed back surface of the tiles. This disadvantage is most troublesome during shipment of the tiles when they are generally stacked, one top of the other, so that the wear or top surface comes into contact with the exposed bituminous backing layer causing some bitumen to stick to the top surface of the tile underneath it and thereby ruin the tile even before it is installed. Typically, in order to overcome this deficiency, it has been thought to be necessary to provide on the back of the carpet tile a secondary backing sheet as disclosed, for instance, in U.S. Pat. No. 4,702,950 (see Column 4, lines 30 to 41). The use of such backing sheet, however, increases the cost of the carpet-tile product thereby eliminating or obviating the cost advantage of a bituminous backing system and does little or nothing to enhance the performance of the carpet tile in actual use. In fact, the presence of such an additional layer in the carpet tile may offer the opportunity for product failure during use due to delamination.
Carpet backing compositions composed predominantly of amorphous polyolefins are described in European Patent No. 309,674, published Apr. 5, 1989. The patent also discloses the use of polyolefins which have been reacted with unsaturated acid or acid anhydrides in the backing composition.